In some prior art printers, image pixels are formed on photosensitive media by means of optical energy (e.g., a light beam). One source of this optical energy may be a LED (Light Emitting Diode) housed in the print head, which may also house a focusing lens for focusing the optical energy onto the phtosensitive media. A carriage carrying the print head translates linearly along one dimension of the photosensitive media as the photosensitive media is held momentarily stationary beneath the print head, whereupon the print head prints one or more lines of image data on the photosensitive media. After one sweep of the carriage, the photosensitive media is advanced a predetermined distance and another sweep is performed to print another line of image data on the photosensitive media. By modulating the image data in synchronization with translation speed of the photosensitive media, a complete raster image is eventually printed or exposed onto the photosensitive media.
A typical non-contact LED (Light Emitting Diode) array image printer is disclosed in U.S. Pat. No. 4,837,589 titled "Non-Contact LED-Array Image Printer" issued Jun. 6, 1989 in the name of Dennis W. Dodge. This patent discloses an LED array mounted on a substrate bearing an interface control circuit which receives video data through a ribbon cable. The LED array is imaged by a lens onto an exposure plane on a platen parallel to the direction of scanning. A photosensitive medium is driven in registration in forward and reverse directions biased against the exposure platen which defines the exposure plane. However, it appears that one limitation of this device is that it produces only one size of pixel. Therefore, in order to produce images of multiple pixel resolutions, it is necessary to write pixels of smaller size multiple times to produce larger pixels. This results in lowered printing speed.
Therefore, there has been a long-felt need to provide an apparatus and method of forming multiple image pixel sizes on photosensitive media, which apparatus and method avoid the need to write smaller pixels multiple times to produce pixels of larger size.